In the prior art, lower carbon, high strength (or High Performance Steel, HPS) weathering grade steels are being increasingly employed for bridge, pole and other high strength applications. These steel materials offer three advantages over concrete and other types of steel materials. First, the use of higher strength materials can reduce the overall weight of the structure being built and can also reduce the material cost. Consequently, designs using these weathering grade steels can be more competitive with concrete and those designs employing lower strength steels. Second, the weathering grade or atmosphere corrosion-resistant grade steel can significantly reduce the maintenance cost of structures such as bridges or poles by eliminating the need for painting. These weathering grade steels are particularly desirable in applications which are difficult to regularly maintain, for example, bridges or poles located in remote areas. Third, lower carbon (i.e., 0.1% maximum) and lower carbon equivalent levels improve the weldability and toughness of the steel.
The use of these types of steels is guided by ASTM specifications. For a medium strength application, e.g., ASTM A588-Grade B or A709-Grade 50 W, weathering steels having a 50 KSI minimum yield strength are specified. These steels typically employ about 0.16% by weight of carbon.
Other ASTM specifications for weathering steels which are commonly used for bridge and pole applications include A709-Grades 70 W and HPS 70 W for bridge applications, and A871-Grade 65 for pole or tubular applications. The bridge-building, 70 W grades require a 70 KSI minimum in yield strength. The specification requires that these grades be produced by rolling, quenching, and tempering. The conventional 70 W grade is a higher carbon grade (0.12% by weight), whereas the newer HPS 70 W grade utilizes a lower carbon level (0.10% by weight). The HPS 70 W grade is generally produced in plates up to 3"in thickness. Table 1 lists the ASTM specifications with Table 2 detailing the mechanical property requirements for the various specifications. Table 3 details the compositional requirements for these specifications. The disclosure of ASTM specification numbers A871, A852, A709 and A588 are hereby incorporated by reference. As noted above, the higher strength specifications require a hot rolled, quenched, and tempered processing. Moreover, the tensile strength is specified as a range, i.e., 90-110 KSI, rather than a minimum which is used in other specifications, see for example, A871-Grade 65 that specifies a tensile strength greater than or equal to 80 KSI.
These high strength ASTM specifications are not without their disadvantages. First, processing whereby the hot rolled, quenched and tempered product is energy intensive. Second, these quenched and tempered grades are limited by plate length due to furnace length restrictions. In other words, only certain length plates can be heat treated following the quenching operation since the furnaces will accept only a set length, in some instances, only up to 600". Bridge builders particularly are demanding ever-increasing lengths (to reduce the number of splicing welds required and save fabrication cost) of plate for construction; such demands are not being met by current plate manufacturing technology for high strength steels.
Third, the high strength ASTM specifications requiring a minimum of 70 KSI yield strength also pose a difficulty by specifying an upper limit for tensile strength, i.e., 110 KSI for A709-Grade 70 W. More particularly, one cannot merely target a minimum 70 KSI yield strength to meet the A709 specification since too high of a yield strength may also result in a tensile strength above the 110 KSI maximum.
In view of the disadvantages associated with current high strength weathering grade steel specifications, a need has developed to produce plates in ever-increasing lengths and in a more cost-effective manner (lower production cost and quicker delivery). In addition, a need has developed to provide a method for making a multi-purpose plate product that meets a number of different ASTM specifications with a single alloy chemistry and/or processing sequence. Such a development would allow longer caster strings and grade consolidation, improve production yield, and reduce slab inventory.
In response to the above-listed needs, the present invention provides a method of making a multi-purpose weathering grade steel plate and a product therefrom. More particularly, the inventive method uses a controlled alloy chemistry, a controlled rolling, and a controlled cooling to produce an as-rolled and cooled weathering grade steel plate which meets a number of ASTM specifications in terms of compositional and mechanical property requirements. The inventive method combines controlled rolling and accelerated cooling with the controlled alloy chemistry to meet the ASTM specifications for 65 KSI and 70 KSI minimum yield strengths and plate thicknesses up to 1.5" and 1.25", respectively. The processing is more energy efficient since no re-austenitizing and tempering are required.
The use of accelerated cooling and hot rolling is disclosed in U.S. Pat. No. 5,514,227 to Bodnar et al. (herein incorporated in its entirety by reference) This patent describes a method of making a steel to meet ASTM A572, Grade 50, a 50 KSI minimum yield strength specification. The alloy chemistry in this patent specifies low levels of vanadium and 1.0 to 1.25% manganese. Bodnar et al. is not directed to weathering grade steels nor methods of making plate products requiring yield strength in the range of 65 to 70 KSI.